Implantable electrode arrays

ABSTRACT

An implant stimulator device uses tantalum and tantalum pentoxide as a system for the conveyance of electrical stimulation pulses from stimulus-forming circuitry contained within an hermetic enclosure to the saline fluids of body tissue to be stimulated. Internal coupling capacitors are not used, yet the danger of having DC current flow to the saline fluids is eliminated. A preferred embodiment provides a multiplicity of electrode contacts made from sintered, anodized tantalum, connected via tantalum wire leads to tantalum feedthroughs into the hermetically sealed package containing the stimulus pulse-forming electronic circuitry. One or more counter electrode contacts (for monopolar or bipolar configurations, respectively) made of activated iridium, non-activated iridium, iridium in combination with a noble or non-noble metal, platinum, gold, or other metal which forms a low impedance contact with body fluids, is/are connected via platinum or other conductive metal leads to return feedthroughs. When powered-up, the stimulus generating circuit produces a steady polarizing potential of approximately half its maximum output voltage range, which potential is applied as a positive (anodizing) voltage to each tantalum electrode and associated lead and feedthrough, with respect to the counter electrode(s), which act as the reference point for the circuit.

This application is a continuation-in-part of U.S. application Ser. No.08/784,209, filed Jan. 15, 1997, now U.S. Pat. No. 5,833,714.

BACKGROUND OF THE INVENTION

The present invention relates to implantable stimulation devices, e.g.,cochlear prostheses used to electrically stimulate the auditory nerve,or other nerve or muscle stimulators, and more particularly to theelectrodes or electrode contacts used with such implantable stimulatingdevices.

Most cochlear prosthesis currently employ an array of closely spacedelectrode contacts implanted in the scala tympani, where they are usedto selectively stimulate multiple regions of the tonotopically arrangedspiral ganglion cells, which comprise the auditory nerve. In order tominimize the electrical power required to activate the nerve cells, theimpedance of the electrical load presented to the stimulating circuitryneeds to be minimized. In order to avoid damage to the neurons and othersurrounding tissues, the stimulation waveforms must be strictlycharge-balanced; i.e., there must be no net direct current flow. Inorder to simplify surgical implantation and minimize danger of extrusionthrough the skin, the volume occupied by the electronic circuitry andany associated packaging and connections must be minimized.

In the present art, the above requirements are usually met by employingmetal electrode contacts, and connecting such electrode contacts tohermetically sealed electronic circuits via leads and feedthroughs intothe hermetically sealed electronic package. The electrode contactstypically consist of the noble metal platinum and its alloys withiridium. This choice gives rise to the following design problems andstrategies for coping with them:

1. Provision of Biphasic Pulses

Electrical stimulation pulses having two opposite phases each with equalcharge are actively applied to each contact. Sometimes such pulses areapplied to one intracochlear contact with respect to an indifferentelectrode usually located outside the cochlea, a configuration known as"monopolar." At other times or in other devices, such pulses may beapplied bipolarly between two or more intracochlear contacts. Ingeneral, these actively driven biphasic pulses require pulse-formingcircuitry to produce two different polarities of voltage and current,either by operating in a push-pull configuration with both positive andnegative power supply voltages or by employing switching networks in theoutput stages that can reverse the direction of current flow through theelectrodes to which it is intermittently connected. When such circuitryis used with more than one output circuit and/or electrode pair at atime, care must be taken to avoid inadvertently producing summation ofoutput voltages with voltages stored on various output capacitors andelectrodes that may exceed the operating voltage range of the circuitry,thereby producing unreliable function or damage. In practice, this oftenmakes it impossible to use the full dynamic range of voltage that isactually produced or nominally tolerated by the circuitry.

2. Prevention of Net DC

Usually a capacitor is incorporated in series with each intracochlearcontact to block any net DC current that might arise through slightinequalities between the two opposite phases of the stimulation pulses.One disadvantage of such capacitors is that they have a finiteimpedance, which is in series with the electrode impedance, andincreases the voltage that must be provided by the output circuit inorder to produce a given output current in the total load. This can beminimized by using a large capacitance value component, but suchcapacitors are physically bulky and one is required for eachintracochlear electrode contact.

In order to save space in some cases, blocking capacitors have beenomitted in favor of a passive discharge scheme which assumes that anyresidual charge will be trapped by and produce a small polarization ofthe intrinsic capacitance of the metal-electrolyte interface, whichpolarization can be bled off the capacitance by shorting the electrodepair together between pulses. The maximal permissible polarization thatcan accumulate safely on the metal-electrolyte interface is limited bythe electromotive force required for the electrolysis of the saline bodyfluids, which is about ±0.8 VDC, a value that is far lower than thecompliance voltage actually available to the electronic circuitry (±5 to±15 VDC). Thus, the stimulus-generating circuitry can easily produceconditions that are known to lead to electrolytic damage to both theelectrodes and the surrounding tissue.

3. Prevention of Shunts Between Leads

Each of the intracochlear contacts generally requires a separateinsulated wire from the stimulus generating circuitry to the contactitself. These fine wires must be bundled closely together in a highlyflexible cable and they often sustain substantial handling duringfabrication of the electrode array and cable. Because the electrodesthemselves have substantial impedance, even small leaks in theinsulation between individual wires and between a wire and theconductive body fluids surrounding the lead and the electrode may resultin substantial shunting of output current away from the desiredelectrode contact, thereby degrading performance.

4. Prevention of Shunts Between Feedthroughs

The electronic circuitry is usually sealed into an enclosure made ofceramic and/or metal with hermetically sealed feedthroughs to each ofthe leads going to each of the electrodes. In order to minimize the sizeof the case, it is desirable to make these multiple feedthroughs assmall and closely spaced as possible. However, the feedthroughs and theattachments to the wire leads constitutes one of the most vulnerablepoints for the development of electrical shunts. This is because watervapor that is present in polymeric encapsulants placed over theseconnection points outside the hermetic package tends to condense on thehydrophilic metal and/or ceramic surfaces of the package and thedielectrics that form part of the feedthrough assemblies. Condensedwater tends to dissolve ions from these surfaces and enlarge in volumeunder osmotic pressure, forming conductive shunts between feedthroughsand leads.

It is thus apparent that there is a need in the art for implantableelectrodes for use with an implantable electronic device whichfacilitate the use of biphasic pulses, prevent the flow of net DCcurrent, and prevent shunts between leads and feedthroughs.

The use of sintered, anodized tantalum as a bioelectrode has beenproposed in the prior art (Guyton & Hambrecht, ca 1973), but it has yetto be incorporated into a medical product. A suggestion has been made,see Loeb, et al. (1991, Med. & Biol. Engng. & Comput. 29:NS13-19) to usesuch electrodes to store the energy for stimulation pulses in anelectrolytic capacitor formed by sintered, anodized tantalum in serieswith saline body fluids and a counter electrode made of iridium with anelectrochemically activated surface. See also , U.S. Pat. Nos.5,193,539; 5,193,540; 5,312,439; 5,324,316 and 5,405,367, which patentsare incorporated herein by reference. The combination of electrodesdescribed in these references is particularly useful for the generationof monophasic pulses because tantalum pentoxide protects against leakagecurrent only as long as it is held at a neutral or anodic potential.Tantalum pentoxide cannot be reverse-biased by more than about 1 volt.Iridium, on the other hand, is equally resistant to corrosion in eitherpolarization. Furthermore, iridium can be electrochemically activated,if necessary, resulting in a surface layer of iridium oxide that tendsto float rapidly to neutral polarization regardless of changes in thepolarization of the tantalum pentoxide (Loeb, et al., 1991).

Tantalum metal is a metal of choice for the hermetic feedthroughs andwire leads because tantalum metal is strong, ductile, highly conductive,biocompatible, inexpensive and readily drawn into wire of any desireddimension. It spontaneously forms a native oxide that facilitateshermetic sealing to various glass and ceramic dielectrics commonlyemployed in hermetic feedthroughs. Use of anodized tantalum feedthroughsand wire leads with non-tantalum electrodes in an implantable electronictissue stimulator was described by White, 1980 (Annals of Biomed. Engng.8:317-332).

However, using tantalum (Ta) for the electrode material still leavesunanswered the question of what type of material can or should be usedas a counter-electrode. Heretofore, it has been taught that an activatediridium (Ir) electrode should be used as the counter-electrode with a Taelectrodes because of its non-polarizing nature. However, this places asignificant restraint on the commercial feasibility of any suchimplantable electrode array because Ir is expensive, difficult tofabricate, and cumbersome to activate during manufacture.

Thus, it is evident that improvements are still needed in implantableelectrodes, and in particular in the type of counter electrode that isused with a Ta electrode system.

SUMMARY OF THE INVENTION

The present invention addresses the above and other needs by providingan implant device that uses tantalum and tantalum pentoxide as anelectrode system for the conveyance of electrical stimulation pulsesfrom stimulus-forming circuitry contained within an hermetic enclosureto the saline fluids of the body tissue to be stimulated in combinationwith at least one counter electrode made from one or more specifiedmaterials, such as iridium (activated or non-activated); iridium incombination with other noble metals, such as platinum (Pt), aplatinum-iridium (PtIr) alloy, or non-noble metals, such as Ta itself;or other metals, such as platinum or gold, which form stable,low-impedance contacts with body fluids.

Preferred applications for the present invention include a cochlearprosthesis or an implantable microstimulator or nerve/muscle stimulator.Such prostheses comprise of a multiplicity of electrode contacts madefrom sintered, anodized tantalum, connected via tantalum wire leads totantalum feedthroughs into an hermetically sealed package containingstimulus pulse-forming electronic circuitry. One or morecounter-electrode contacts (for monopolar or bipolar configurations,respectively) are made from a specified metal or combination of metals,connected via platinum or other metal leads to metal returnfeedthroughs. When powered-up, the stimulus generating circuit producesa steady polarizing potential of approximately half its maximum outputvoltage range, which potential is applied as a positive (anodizing)voltage to each tantalum electrode and associated lead and feedthrough,with respect to the counter electrode(s), which act as the referencepoint for the circuit.

In accordance with one aspect, the present invention advantageouslyeliminates the need for internal coupling capacitors without the usualdanger of having DC current flow to the saline fluids. Further, thepresent invention eliminates the need for high integrity and adhesion inother dielectrics used to encapsulate the feedthroughs and wires outsidethe hermetic package en route to the cochlea (or other stimulationsite).

When used with an appropriate counter electrode and stimulus-formingcircuitry of a type described herein, the present invention increasesthe power efficiency, the available compliance voltage, and thereliability of such stimulus-forming circuitry. Moreover, the presentinvention simplifies the fabrication and improves the reliability of theelectrode array and associated flexible cable connecting the array tothe stimulus-forming circuitry.

It is thus an object of the present invention to provide an electricalimplant device that avoids the use of coupling capacitors, yeteliminates the danger of applying DC current to saline fluids/tissue.

It is a further object of the invention to provide implantableelectrodes, or an implantable electrode array, that employs tantalummetal.

It is another object of the invention, in accordance with one embodimentthereof, to provide an implantable stimulation device wherein tantalumand tantalum pentoxide combine to provide an electrode system for theconveyance of electrical stimulation pulses from hermetically sealedstimulus-forming circuitry to the saline fluids/tissue to be stimulated.

It is yet an additional feature of the invention, in accordance withother embodiments thereof, to provide an implantable counter electrodefor use with a Ta electrode system that need not be made exclusivelyfrom activated iridium, but which can be made from non-activated iridiumand/or iridium in combination with other metals, or from other metals.

It is a feature of the invention to reduce the cost, simplify thedesign, and improve the reliability of implantable electronic devices,including implantable electrode arrays.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other aspects, features and advantages of the presentinvention will be more apparent from the following more particulardescription thereof, presented in conjunction with the followingdrawings wherein:

FIGS. 1A-1D diagrammatically depict various electrode arrays made inaccordance with the present invention;

FIG. 1E shows an enlarged view of a tantalum electrode used in theembodiments of the invention shown in FIGS. 1A-1D, and illustrate theformation of a tantalum pentoxide layer on the electrode;

FIG. 2 is a block diagram illustrating how the electrodes of theinvention are used with stimulus-forming circuitry;

FIGS. 3A and 3B are timing diagrams that illustrate the voltage andcurrent, respectively, associated with the use of the electrodes andstimulus-forming circuitry of FIG. 2;

FIG. 4 shows alternative stimulation circuitry that may be used inaccordance with the invention when only biphasic stimulation pulses aredesired;

FIGS. 5A and 5B show current and voltage waveforms associated with theoperation of the circuit of FIG. 4;

FIG. 6A shows a radial cross sectional-view of an electrode array madein accordance with the invention, with the view being made near the baseof the array; and

FIG. 6B shows a longitudinal cross-sectional view of the electrode arrayof FIG. 4A, with the view being taken near the apex of the array.

Corresponding reference characters indicate corresponding componentsthroughout the several views of the drawings.

DETAILED DESCRIPTION OF THE INVENTION

The following description is of the best mode presently contemplated forcarrying out the invention. This description is not to be taken in alimiting sense, but is made merely for the purpose of describing thegeneral principles of the invention. The scope of the invention shouldbe determined with reference to the claims.

It is noted that the detailed description which follows is directed to acochlear prosthesis. However, the invention is not limited to use with acochlear prosthesis, but may be used with any type of implantablestimulator, e.g., a nerve stimulator, a pain stimulator, a musclestimulator, or the like.

The general design of a cochlear prosthesis comprises of a multiplicityof electrode contacts made from sintered, anodized tantalum, connectedvia tantalum wire leads to tantalum feedthroughs into the hermeticallysealed package containing the stimulus pulse-forming electroniccircuitry. One or more counter electrode contacts (for monopolar orbipolar configurations, respectively) have heretofore consisted ofactivated iridium, connected via platinum or other noble metal leads tonoble metal feedthroughs. However, as explained below, it has recentlybeen learned that the counter electrode contacts may also be made fromother materials. When powered-up, the stimulus generating circuitproduces a steady polarizing potential of approximately half its maximumoutput voltage range, which potential is applied as a positive(anodizing) voltage to each tantalum electrode and associated lead andfeedthrough, with respect to the activated iridium electrode(s), whichact as the reference point for the circuit. This powering-up processmust be conducted slowly enough so that the currents required for thebuild-up of charge on the Ta electrodes do not produce undesirablestimulation or electrochemical effects; similarly, the powering-downprocess of discharging the Ta electrodes must be well-controlled.Stimulus pulses or other desired biphasic waveforms are produced bydriving each tantalum electrode more or less positive than this nominalsteady polarizing potential.

The available capacitance of the sintered, anodized Ta electrode needsto produce a reactance to the applied current that is small compared tothe rest of the impedance of the load, which consists largely of theresistance of the surrounding saline-filled tissues. A capacitance ofabout 0.1-1.0 μF is readily attainable, which produces a reactance ofless than 1 kΩ compared to the tissue resistance of about 10 kΩ. Thesteady anodizing potential applied to the tantalum components tends toreinforce the previously applied anodization layer, limiting to anominal value the amount of DC that can be applied even in the event ofa failure of the circuitry or a defect in the anodization layer. Infact, portions of the tantalum leads that are generally well-covered byother dielectrics do not need to be anodized prior to use because theprolonged presence of the anodizing polarization voltage will eventuallycause any small regions of exposed tantalum to become anodized. Thecathodic potential applied to the iridium electrode is absorbed byvalence shifts within the activated iridium layer, as described by Loeb,et al. (1991).

As shown in FIG. 1A, the implanted portions of the cochlear prosthesiscomprise an hermetically sealed enclosure 20 containing stimuluspulse-generating circuitry (P-6 CKT) 100. A flexible cable 40 containselectrical leads 45 that connect the circuitry 100, via feedthroughs 25,to an intracochlear electrode array 60, all disposed in saline fluids 5of the body tissues in which these portions are surgically implanted.The intracochlear electrode array 60 contains a multiplicity ofsintered, anodized tantalum electrode contacts 65, each connected via ananodized tantalum lead 45 to an anodized tantalum feedthrough 25 thatpasses through and is hermetically sealed to the enclosure 20. One ormore counter electrodes 75 are disposed within the electrode array 60 asshown in FIG. 1A and FIG. 1B. Alternatively, or additionally, as shownin FIG. 1C or 1D, one or more counter electrodes 75 may be locatedoutside of the electrode array 60. Typically, as taught in applicants'related application, Ser. No. 08/784,209, filed Jan. 15, 1997, now U.S.Pat. No. 5,833,714 (hereafter the "parent" application), each suchcounter electrode 75 is connected via a noble metal conductor 55 to anoble metal feedthrough 35 of the enclosure 20. If it is desired to havethe counter electrode 75 located in a particular manner within thecochlear array as shown in FIG. 1A, or if more than one counterelectrode 75 is employed as shown in FIG. 1B, then the correspondingconductor(s) 55 and feedthroughs 35 may be insulated with a conventionaldielectric coating 10 such as silicone elastomer or Teflon. The anodizedtantalum components 25, 45 and 65 require no additional insulationbeyond the electrochemically grown layer of tantalum pentoxide 15 (FIG.1E) that is formed on their surfaces during manufacture or during use.However, the application of dielectric coatings, encapsulates ormoldings 10 may be desirable to limit the passage of alternating currentfrom stimulation pulses between the tantalum conductors and into anysurrounding saline fluids as a result of stray capacitance through thetantalum pentoxide 15. Generally, some such coatings, encapsulates ormoldings 10 will be employed to provide mechanical strain relief for theconnection of wire leads to feedthroughs and to give the desiredphysical shape and handling properties to the electrode array 60 andflexible cable 40.

When the above parent application was filed, it was thought, and indeedall of the prior art teachings had been, that the counter electrode(s)75 had to be made from activated iridium. However, applicants haverecently discovered that this may not be so. That is, applicants havediscovered that if iridium (Ir) is used as the counter electrode, itprobably is not necessary to activate it explicitly during manufacturebecause it has a native oxide that provides some activation. Moreover,this native oxide advantageously tends to grow spontaneously during deepvoltage cycling if and when the electrode is used to pass stimulationcurrents. Thus, in essence, the electrode appears to be self-tuning,with little activation present or needed during light stimulation andmore activation developing spontaneously to handle intense stimulation.

Additionally, applicants have learned that if Ir is used as the counterelectrode, it is not necessary for it to be the only metal present andin contact with the body fluids. Rather a combination of metals, with Iras one of the metals, may be used. For example, it is permissible forpart of the electrode surface to be made from another noble metal suchas Pt or a PtIr alloy. Moreover, applicants have recently discoveredthat other, non-noble metals such as Ta itself may be used. Thisprovides a significant advantage for some designs, such as themicrostimulator design disclosed in U.S. Pat. Nos. 5,324,316 or5,358,514, or similar designs which incorporate a glass or ceramichermetically sealed capsule or housing wherein the electronicstimulation circuits are housed. This is because using Ta for thecounter electrode enables the use of Ta-glass (or Ta-ceramic) seals forboth electrode feedthroughs (the one used with the electrode and the oneused with the counter electrode) needed to make a complete electricalpath with the electrical stimulus circuitry housed within the hermeticglass/ceramic capsule.

What appears to happen when Ta or another non-noble metal is used as thecounter electrode is that the counter electrode is under continuouscathodal protection from corrosion as a result of the anodal charging ofthe Ta. Even deep stimulation pulses do not drive the counter electrodesufficiently positive to lead to corrosion or oxidation of the non-noblemetal. The main requirement is that the combination of metals not leadto spontaneous dissimilar metals corrosion when the implant is not underpower.

Applicants have also recently discovered that it may not be necessary touse Ir at all in the counter electrode. Rather, metals such as platinumor gold that form stable, low-impedance contacts with body fluids underpassive conditions are also probably adequate, if not ideal, as counterelectrodes in the Ta systems because of the above-noted cathodalprotection. That is, while an Ir counter electrode theoretically offersa lower interface impedance, when a sufficiently large surface area isused to allow the desired charge per pulse to be injected, other metalswill be adequate, particularly where the cathodal bias prevents orreduces oxide formation. For many of the applications of interest, e.g.,a cochlear electrode or a microstimulator, the circuit impedance isdominated by the surrounding tissue, not by the metal-electrolyteinterfaces. Thus, metals other than iridium may be used as the counterelectrode.

A significant advantage of being able to use other metals, e.g. platinumor gold, for the counter electrode is that it now becomes possible,using the Ta electrode system of the present invention, with itscathodal protection feature, to use very thin metal casings for thehermetic enclosure or housing, allowing the case itself to be used as alarge, indifferent counter electrode without danger of leaks arisingfrom pitting corrosion of the case during stimulation or from galvaniccorrosion due to dissimilar metals used in brazes and other seals andwelds.

Turning next to the timing diagrams of FIGS. 3A and 3B, the electricalcircuitry 100 applies a steady anodic polarization +Vp during periods110 to each tantalum feedthrough 25 and its connected tantalum wire 45and tantalum electrode contact 65, with respect to the reference point(e.g., ground reference) 120 (FIG. 2) of the circuit which is connectedto the noble metal feedthrough(s) 35 and thence to any iridium,iridium/Ta, or other counter electrode(s) 75. This steady anodicpolarization during the period 110 is approximately half of theavailable power supply voltage (sum of +Vp plus +Vs) 105 that isavailable in the electrical circuitry 100. When the application ofstimulating current to the electrode array is desired, during time 115(FIG. 3B), the electrical circuitry 100 produces a sequence of potentialchanges during time 112 (FIG. 3A) in the potential applied to theparticular electrode contacts through which the stimulating current isdesired to flow. The electrode contacts 65 must always be stimulated aspairings in which one or more contacts are connected so as to form onehalf of the pair, all of which contacts must be anodized tantalumelectrode contacts 65, and one or more contacts are connected so as toform the other half of the pair, i.e., the counter electrode, all ofwhich counter electrode contacts 75 may be activated iridium contacts,non-activated iridium contacts, iridium in combination with other nobleor non-noble metals, or other metals, as discussed above. When furtherstimulating current is no longer desired, the sequence of potentialvariations or changes about the polarization voltage +Vp (during timeperiod 112) is terminated, with a return to the steady anodicpolarization +Vp during period 110.

Because the anodized tantalum surfaces comprising one half of thepairing act like capacitors with very low DC leakage, onlycharge-balanced alternating current can pass through the saline fluids 5regardless of the potentials applied to these electrodes. If thesequence of voltage variations or changes does not itself result in nonet charge flow in either direction, as shown, e.g., at time 112' (FIG.3A), then the return to the steady anodic polarization +Vp during time110' results in discharge of the charge stored temporarily in thiscapacitance, resulting in a current waveform such as shown at time 115'in FIG. 3B.

All of the coutner electrode contacts 75 are connected to the referencepoint 120 of the electronic circuitry 100, which operates with only onepolarity of voltages +Vs and +Vp (105) in its power supply. Thus, theelectronic stimulus-forming circuitry 100 is not capable of causing netdirect current to flow through the counter electrode contacts 75.

Thus, in operation, it is seen from FIGS. 2, 3A and 3B that thestimulus-forming circuitry derives a source of operating potential Vs,which it uses as a source of power within the stimulus-formingelectronic circuitry. There is also included within, or coupled to, thestimulus-forming circuitry 100 a means for maintaining an anodic voltage+Vp between the tantalum feedthrough(s) 25 and the noble metalfeedthrough 35, with the tantalum feedthrough being positive relative tothe noble metal feedthrough. Any suitable means may be used formaintaining or applying this anodic voltage +Vp between thefeedthrough(s) 25 and feedthrough 35, as is known in the art. Note thatthe anodic voltage +Vp is generally no greater than about 1/2 of theoperating potential Vs.

When a biphasic stimulation current pulse is desired, the voltage thatis applied between the tantalum feedthrough(s) 25 and the noble metalfeedthrough 35 by the stimulus-forming circuitry 100 is varied by anamount ±Δ, where the voltage amount Δ is that amount required to causethe desired stimulating current to flow through electrode 65, butkeeping such applied voltage within the range of zero to the sum of +Vpand +Vs. That is, as seen best in FIGS. 3A and 3B, the voltage appearingacross the tantalum feedthrough(s) 25 and the noble metal feedthrough 35comprises a voltage +Vp-Δ for a period of time from t1 to t2, followedby a voltage +Vp+Δ for the period of time t2 to t3. It is generallypreferred that the time period t1-t2 be the same as the time periodt2-t3 so that the resulting biphasic current pulse is balanced. Ofcourse, balancing may still be achieved even if the time periods t1-t2and t2-t3 are not equal simply by adjusting the amplitudes of therespective negative or positive halves of the biphasic pulse.

It should also be noted that while it is generally preferred that thefirst half of the biphasic pulse be negative, as shown in FIG. 3B, suchis not mandated by the present invention. It is possible to have thebiphasic pulse be positive for the first half, followed by the negativepulse. What is important for purposes of the present invention is thatthe voltage appearing across the tantalum feedthrough(s) 25 and thenoble metal feedthrough 35 be varied or changed so that it comprises avoltage +Vp±Δ for a period of time from t1 to t2, followed by a voltage+Vp±Δ for the period of time t2 to t3 so that a balanced biphasiccurrent pulse will flow during the time period t1 to t3 (112) throughthe saline liquids and tissue adjacent the tantalum electrode(s) 65 andthe iridium counter electrode 75.

It should further be noted that the cochlear electrode array andassociated stimulation circuitry that are the subject of the presentinvention can also be used to generate more complex stimulus waveformshaving multiple or even continuous phases, subject only to thelimitation imposed by the capacitive coupling that prevents creatingwaveforms with a net direct current flow in either direction.

In the event that only biphasic stimulation pulses are desired, thecochlear electrode array that is a part of the present invention can beused advantageously with alternative stimulation circuitry 101 shown inFIG. 4. The alternative stimulation circuitry 101, when used inconjunction with an electrode array of the type herein described,produces only biphasic pulses of the form shown in FIGS. 5A and 5B, inwhich the first phase 117 of the pulse applied by tantalum electrode 65is cathodal (i.e., positively charged current flows toward the activetantalum electrode 65 from the reference counter electrode 75). Thispolarity of biphasic pulsing is known to be more effective forstimulating excitable tissues that are physically close to the electrodecontact whose first phase is cathodal. It is also a property of thiscircuitry that the shapes of the successive cathodal phase 117 andanodal phase 118 are not necessarily symmetrical but that the totalcharge delivered (which is the area under the curve in the current vs.time traces i_(e) in FIGS. 5A and 5B) is equal and opposite in the twosuccessive phases 117 and 118.

Circuit 101 operates in conjunction with a single power supply 105 whichis normally connected via switch 107 to tantalum electrode 65 so as topolarize potential Ee to the maximal positive voltage +Vs. Electrodes 65and 75 and the intervening body fluids have an approximately equivalentcircuit 70 composed of the series combination of electrolyticcapacitance 67 and resistance 68. When a stimulation pulse is desired,switch 107 is connected for the duration of the first, cathodal phase117 of the stimulation pulse to a stimulus control circuit 102. At theend of cathodal phase 117, switch 107 reconnects electrode 65 to powersupply 105, causing anodal current 118 to flow as capacitance 67 isrepolarized to the value +Vs. Because of capacitance 67, the chargetransferred during cathodal phase 117 must be equal and opposite to thattransferred during anodal phase 118.

In one alternative embodiment, stimulus control circuit 102 may performthe function of permitting a regulated current to flow. In that case,the strength of the stimulation pulse is determined by controlling theamplitude and duration of the current. This results in the waveforms ofelectrode current i_(e) and potential Ee shown in FIG. 5A. Duringcathodal phase 117, which lasts from t1 to t2, circuit 102 pullspotential Ee progressively lower in order to maintain constant currentflow as capacitance 67 is discharged. During anodal phase 118, from t2to t3, the current i_(e) varies exponentially according to the timeconstant determined by capacitance 67, resistance 68 and any internalresistance of other conductors and power supplies in the circuit.

Alternatively, the strength of a stimulation pulse may be determined bycontrolling the total charge of its cathodal phase 117, withoutexplicitly controlling either the current or the duration. FIG. 5B showsone possible set of waveforms of the electrode current i_(e) andpotential Ee that can be generated in this manner. During cathodal phase117, from t1 to t2, circuit 102 effectively short-circuits tantalumelectrode 65 to reference electrode 75, causing Ee to fall to zero.Current i_(e) flows according to the time constant determined bycapacitance 67, resistance 68 and any internal resistances. After therequired cathodal charge has flowed, from time t2 to t3, circuit 102goes into an open state in which no current flows, during which Eerepresents the residual charge remaining on capacitance 67. Anodal phase118 is produced by reconnecting electrode 65 to power supply 105. Thismethod of controlling stimulus intensity by metering charge instead ofcurrent has the advantage of eliminating the dissipation of power thatnormally occurs in current-regulating circuits. It generates theshortest possible duration of cathodal pulse that can be produced giventhe available power supply voltage +Vs and impedance of equivalentcircuit 70. It permits a delay interval between the cathodal and anodalpulses, which is known to improve the efficiency of the neural responseto electrical stimulation with brief, biphasic pulses.

Turning next to FIGS. 6A and 6B, details are shown of a more completeand explicit design for a cochlear electrode array 200 that incorporatesseveral features of the present invention plus additional features ofinventions disclosed in pending patent applications. For example,reference is made to U.S. patent application Ser. No. 08/516,758, filedAug. 18, 1995, now U.S. Pat. No. 5,649,970 and U.S. patent applicationSer. No. 08/447,455, filed May 23, 1995, both of which patentapplications are incorporated herein by reference, which disclosevarious features of electrodes and electrode arrays for use withimplantable stimulation devices. FIG. 6A shows a transverse crosssectional-view of the electrode array 200 with the sectional view beingmade near the base of the array. FIG. 6B shows a longitudinalcross-sectional view of the electrode array 200, with the sectional viewbeing taken near the apex of the array.

As seen in FIGS. 6A and 6B, the array 200 is preferably made from asilicone molding 202 with a complex cross-section and a performed spiralshape that is designed to optimally position a set of 24 sintered Tacontacts 206 at regular intervals of about 0.8 mm over most of the 24 mmlength of the array. Each contact faces the medial wall of the scalatympani and is surrounded by a very thin, deformable ring 208 of moldedsilicone that acts like a gasket to force its stimulating current intothe bone overlying the spiral ganglion cells. These contacts and gasketsare forced against the medial wall by the combined pull of the springyspiral shape and the push of a longitudinally disposed fin 204 directedtoward the lateral wall.

Two elongated common electrodes 210, 212 are shown above and below thehorizontal meridian and facing laterally. One or the other of theseelectrodes 210 or 212 would be electronically switched to the referencepoint 120 of the circuitry to act as the return electrode in a bipolaredge-effect array as described in the '758 patent application referencedabove. Note that the edge-to-edge distance of the Ir to adjacent Taelectrodes is, in fact, short enough to produce the desired bipolarfocusing based on edge-effects, even though the contacts actually facein opposite directions. The upper Ir common electrode 210 would be mostuseful when stimulating spiral ganglion cells with a high rate of apicaldendrite survival because it would produce current flow through thehabenula perforata in which such dendrites lie. The lower Ir commonelectrode 212 would be most useful when stimulating spiral ganglion cellbodies directly, as these lie mostly below the horizontal meridian, withaxonal processes projecting downward into the modiolar bone. Note thatbecause of the resting polarization of the Ta at a potential of +Vp,(approximately half the operating voltage range) it is possible to applyeither anodal-first or cathodal-first biphasic pulses in either bipolarconfiguration.

Note further that all of the rigid elements are oriented vertically andstacked in as narrow a space as possible to facilitate flexing of theelectrode only in the axis of the spiral during insertion. Inparticular, the 24 Ta leads connecting to each of the 24 Ta electrodesare organized into two 12-conductor ribbon cables 215, each consistingof 12 0.001" Ta conductors held on 0.003" centers by a Teflon extrusion216. Individual leads from these ribbon cables are stripped of theTeflon carrier 216 by a highly focused laser, which melts the end of theTa wire into a small ball and welds it to the sintered Ta electrodecontact 206. It is important to make such connections without contactingthe Ta parts with another metal (such as might be employed in resistancewelding electrodes), because metallic impurities in the Ta may interferewith the formation of a leak-free anodization layer.

In one possible sequence of manufacture, the Ta leads would be similarlylaser welded to the Ta feedthroughs 25 on the hermetic package and thesubassembly of Ta contacts. The ribbon cables 215 and feedthroughs 25would then be anodized to about 4 times the maximum anticipated workingvoltage (the sum of +Vp plus +Vs) to provide a leak-free anodizationlayer. This subassembly would then be placed in the mold along with theribbon-shaped Ir common electrodes 210, 212. A single injection processwould simultaneously form the silicone material comprising the electrodecarrier 216, the flexible lead cable 45 and the strain-relief 40 overthe feedthroughs 25. Note that any voids or defects in the Teflon of theribbon cable or the silicone material would be inconsequential sourcesof shunts because of the prior or eventual anodization of allpotentially exposed Ta surfaces. Any shunts relative to the Irelectrodes 210, 212, their noble metal leads 55 and feedthroughs 35would also be inconsequential because of the principle of theedge-effect electrode, in which only the region of the common electrodesurface that is immediately adjacent to the active electrode actuallycarries any significant amount of current.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.

What is claimed is:
 1. An implantable tissue-stimulating prosthesiscomprisingan hermically sealed package; implantable stimulus-formingelectronic circuitry housed inside of said sealed package; at least onetantalum feedthrough which provides an electrical path to thestimulus-forming electronic circuitry housed within said package; atleast one return feedthrough through said package that provides a returnelectrical path to the stimulus-forming electronic circuitry housedwithin said package, at least one sintered, anodized tantalum electrodecontact; a tantalum wire lead connecting each tantalum electrode contactto a respective tantalum feedthrough, whereby each tantalum electrodecontact is electrically connected to a respective tantalum feedthrough,and hence to the stimulus-forming electronic circuitry; a layer oftantalum pentoxide covering said tantalum wire lead on at least thoseportions of said tantalum wire lead that are exposed to body fluids; atleast one counter electrode contact, each counter electrode contactcomprising a metal selected from the group consisting of tantalum,non-activated iridium and gold; and a metal lead connecting each counterelectrode contact to a respective return feedthrough.
 2. The implantabletissue-stimulating prosthesis as set forth in claim 1 wherein theimplantable stimulus-forming electronic circuitry includesmeans forderiving a source of operating potential V_(s) for use by thestimulus-forming electronic circuitry; means for maintaining an anodicvoltage +Vp between the at least one tantalum feedthrough and the atleast one return feedthrough, with the at least one tantalum feedthroughbeing positive relative to the at least one return feedthrough, theanodic voltage +Vp being no greater than about 1/2 of the operatingpotential V_(s) ; and means for varying the voltage that is appliedbetween the at least one tantalum feedthrough and the at least onereturn feedthrough by an amount ±Δ so as to cause a balanced biphasiccurrent pulse to flow between the tantalum electrode and the counterelectrode.
 3. The implantable tissue-stimulating prosthesis as set forthin claim 1 wherein said implantable stimulus-forming electroniccircuitry includes:means for deriving a single source of operatingpotential V_(s) and applying said potential V_(s) to maintain an anodicpolarization between the at least one tantalum feedthrough and the atleast one return feedthrough when stimulation is not desired; and meansfor controlling and generating a cathodal phase of a biphasic stimulusby discharging the electrical charge stored by the capacitance of saidat least one anodized tantalum electrode contact, said at least onecounter electrode contact and the intervening tissues and fluids.
 4. Theimplantable tissue-stimulating prosthesis as set forth in claim 3wherein said means for controlling and generating the cathodal phase ofa biphasic pulse produces an approximately constant flow of currenthaving a prescribed magnitude for a prescribed period of time.
 5. Theimplantable tissue-stimulating prosthesis as set forth in claim 3wherein said means for controlling and generating the cathodal phase ofa biphasic pulse acts as a short circuit until a prescribed amount ofcharge has flowed.
 6. An electrical tissue stimulator comprising:anhermetically sealed case; implantable pulse-forming electronic circuitrywithin said case; a plurality of tantalum feedthroughs which providerespective electrical paths to the pulse-forming electronic circuitrywithin said case; a plurality of anodized tantalum electrode contacts; aplurality of tantalum wire leads, each connecting a respective one ofsaid tantalum electrode contacts to a corresponding one of said tantalumfeedthroughs, whereby each tantalum electrode contact is electricallyconnected through its respective tantalum feedthrough to thepulse-forming electronic circuitry within said case; a counter electrodecontact, the counter electrode contact comprising a metal selected fromthe group consisting of tantalum, non-activated iridium and gold; andfeedthrough means for electrically connecting the counter electrodecontact to the pulse-forming electronic circuitry through said case. 7.The electrical tissue stimulator of claim 6 wherein at least a portionof the hermetically sealed case comprises the counter electrode contact.8. The electrical tissue stimulator of claim 6 wherein said means forelectrically connecting the counter electrode contact to thepulse-forming electronic circuitry within said case includes a returnfeedthrough and a conductive lead.
 9. The electrical tissue stimulatorof claim 8 wherein the conductive lead comprises platinum.
 10. Theelectrical tissue stimulator of claim 6 wherein the plurality oftantalum electrode contacts are connected to the pulse-forming circuitrywithin said case without the use of any internal coupling capacitors.